This invention relates to improved data communications in a transmission system having an analog signal transmitter; an analog signal receiver having a threshold element, such as a comparator, for receiving the transmitted signal; and a transmission link directly interconnecting the transmitter and the receiver. In particular, the invention relates to improved data communication in a transmission system using an interface conforming to the Electrical Industry Association (EIA) RS 232 standards as a transmission link.
A number of interfaces that conform to RS 232 standards are available in the commercial market. Such interfaces are well suited to some applications, but their use is limited, in some instances, by maximum cable length specifications. For example, in the above-described transmission system, the RS 232 cable is connected between a transmitter and a receiver having a comparator, which converts the analog signal transmitted by the cable into a "squared-up" digital signal. As the length of the RS 232 cable increases, the reliability of the analog to digital conversion decreases. Therefore, the RS 232 minimum signal transmission quality requirements dictate the maximum, allowable distance between transmitter and receiver. Two factors reducing conversion reliability, and therefore signal transmission quality, are cross-talk and phase margin error, both of which increase as the length of the RS 232 cable increases.
Cross talk, the undesired signals on one signal path that are caused by pulse signals in adjacent signal paths, appears as noise at the input of the comparator. Cross talk, which increases with increasing length of the RS 232 cable can be reduced by shielding the cable or by other equally costly techniques.
Phase margin, the difference in time between the expected and actual occurrence of the phase-delineating edge of the comparator output pulse, is a measure of the extent to which the operation of the comparator has been disrupted. One contributing factor to phase margin is the natural attenuation of high frequencies that occurs when transmitting a signal along an RS 232 cable. This results in a rounding of the transmitted pulse, and as the transmission cable lengthens, the rounding of the transmitted pulse increases. When an analog comparator receives the rounded pulses, a non-zero positive threshold will shorten positive pulses and lengthen negative ones. Increased "rounding" of the pulses on the cable thus worsens the phase margin.
Phase margin is also worsened, for example, when a string of pulses representing a single state, such as a series of "1" or "0" pulses in an NRZ data format, are transmitted along the RS 232 cable for a period of time, and then a pulse of the opposite value is transmitted. The cable, which acts like a capacitor, is fully charged to a maximum voltage by the earlier string, and cannot discharge quickly enough when the opposite state level is sent. As a result, the threshold comparator shortens the digital signal representing the new state and lengthens the pulse representing the old state. Therefore, the actual timing of the rising and falling edges of the digital signal is disrupted, phase margin decreases, and the reliability of the comparator is reduced.
Analog signal detection reliability is further effected when the comparator has a threshold value which is non-zero. Typically, a non-zero threshold is employed to avoid the effects of an open cable. However, a non-zero threshold tends to shorten the output pulse representing one digital state, lengthen the output pulse representing the other digital state, and therefore worsen phase margin.
Objects of the invention are therefore to improve the quality of analog signal transmission along interface cables, to increase the maximum length of transmission cables, to improve cross talk and phase margin effects in signals on transmission cables, and to improve analog signal detection.